Kate Winslet wore Stella McCartney's It dress to the Venice Film Festival earlier this month. In addition to cinching her waist, the dress' sporty top accentuates the actress' athletic shoulders. Photo: Splash News

When Kate Winslet took to the red carpet earlier this month at the Venice Film Festival in a sporty but sexy dress, jaws dropped over her va-va-voom curves. Though the Oscar winner has never been shy about embracing her voluptuous figure, her exaggerated hourglass form was actually courtesy of her dress — a color-blocked Bicolour Octavia number by Stella McCartney.

“It’s like retouching your body with a frock,” says celebrity stylist June Ambrose of the sheath, which retails for $1,595. The figure-flattering number­ — one of the “optical illusion” dresses from the designer’s fall collection — focuses the eye with a contoured front panel while its black sides seem to disappear, creating a deceptively smaller waistline.

Winslet isn’t the only A-lister to have noted the dress’ cunning trickery.

Celebrities such as La La Anthony and Gloria Estefan have both rocked the sexy dress, while “The X Factor” judge Nicole Scherzinger graced the cover of InStyle UK in the shaded wonder. Liv Tyler wore an electric cobalt-and-black version earlier this year.

“This has become one of the biggest dresses of the year,” says Sasha Charnin Morrison, the fashion director of Us Weekly. “It’s a body-conscious dress that is so clever, and it’s from a major designer. All of the groovy girls wear Stella. Everyone wants to wear her now.”

The shape-shifting dress hung in the window of McCartney’s Meatpacking District store at the start of New York Fashion Week this month and sold out days after the shows ended. Currently out of stock on the company’s Web site, the dress is expected to arrive in new shipments by the third week in October.

Meanwhile, fashion lovers who can’t wait (or afford the designer price tag) can take heart — knockoffs are already hitting the market, including an $84 chocolate-and-cappuccino version from Topshop.

Designers like Herve Leger and Alexander McQueen have, in the past, used similar shading techniques. But the Bicolour Octavia is fashioned from a sturdy cotton stretch fabric, which acts like a stylish scuba suit, sucking in all the unsightly bits that plague most female mortals.

“It keeps your meat neat,” explains Ambrose, who is currently taping a makeover show, “Styled by June,” for VH1. “It’s like a girdle, and gives you that svelte silhouette. In my opinion, this is the perfect dress.”

The sultry sheath is a departure for McCartney, who typically makes cool feminine pieces with a heavy nod toward men’s tailoring.

“This dress is so sexually suggestive without being . . . overtly sexual or disrespectful. It’s so elegant at the same time,” says Morrison.

“It just goes to show you,” adds Ambrose, “you can manipulate your body with your wardrobe.”

kirsten.fleming@nypost.com

Create your own illusion

Can’t get your hands on an “optical illusion” dress? Here are four principles to learn from the magical frock — and employ in your own wardrobe.

* “Layer your dark on the outside and wear your bold colors inside, to get that illusion of shading. Cinch a thin sweater with a dark-colored belt,” says stylist June Ambrose.

* To create a slimming panel on the torso, Ambrose suggests using a scarf. “Lay it along the side of the body, drape it over the shoulder and cinch it with a subtle belt,” she says.